The ability to accurately describe oil field reservoir fluids has positive benefits during both the exploration and production phases of a hydrocarbon-containing reservoir. During production, well-specific knowledge of the fluid composition being removed from the reservoir can yield valuable information for both reservoir modeling and management. It would be advantageous to discover sensor technology that can be permanently deployed within a well bore, and can be used to monitor changes in reservoir fluid characteristics over time. Such sensor technology could provide more accurate data for input into reservoir models without the need for disruptive individual well tests. The technology could also be deployed in production logging tools to perform down-hole fluid analysis on an automated basis.
Fluid analysis based on measurement of the fluid's various dielectric properties holds promise as a means for accurately determining both chemical (e.g. composition-related characteristics) and physical (e.g., viscosity, density, temperature, polarity) characteristics of a fluid. Such methodology could provide advantages over conventional analytical techniques such as optical spectroscopy, nuclear magnetic resonance spectroscopy, and resistivity, in that fluids such as down-hole fluids could be characterized in situ using sensors attached to one or more features associated with a hydrocarbon production well (e.g. a well logging tool). Down-hole fluid characteristics might be determined more efficiently and in greater detail by probing the various dielectric properties of the fluid in situ, rather than through the use of conventional analytical techniques which can require well sampling and remote analysis of the sampled fluids and be attended by significant delays in decision making based on analysis results. Fluid analysis based on measurement of a down-hole fluid's various dielectric properties can provide information on molecular composition of fluid components, hydrocarbon chain length of fluid components, the degree to which a hydrocarbon-containing fluid is saturated or unsaturated, and the polarity of the fluid among others. Because fluid analysis based on measurement of the fluid's various dielectric properties does not involve optical spectroscopy, sensor fouling is less problematic than in down-hole fluid monitoring systems involving optical probe techniques.
Traditionally, impedance spectroscopy, a known form of dielectric fluid analysis, has been applied to characterize fundamental aspects of materials performance. In impedance spectroscopy, a material is positioned between the electrodes of a sensor and is probed over a wide frequency range, from a fraction of a Hertz (Hz) to tens of gigahertz (GHz). While impedance spectroscopy is a useful tool in materials characterization, cell analysis, and particle sizing, its applicability in practical sensors for detection of trace levels of analytes is limited by low sensitivity in known measurement configurations and prohibitively long acquisition times over the broad frequency ranges involved.
Resonant sensors have proven useful in chemical, physical, and biological sensing applications. Resonant sensors based on inductor-capacitor-resistor (LCR) structures with multivariable analysis of the resonance spectra produced, have not been shown to be useful in down-hole environments so critical to hydrocarbon exploration and production activities.
Thus, there remains an unmet need in the areas of chemical, biological, and physical detection for sensors which may be employed in down-hole environments and which offer a combination of high sensitivity, favorable signal-to-noise ratio, high selectivity, high accuracy, and high data acquisition speeds.